The invention described herein relates generally to structures and methods for measuring solar radiation and more particularly to structures and methods for measuring photosynthetically active radiation received in a vegetative canopy.
Both ecologically and agriculturally, the penetration of solar radiation into vegetative canopies is an important process. Because the process is so highly stochastic both in space in time it is difficult to measure with accuracy, unless flux measurements are averaged over space or time, as by employing numerous line sensors. However, averaged flux densities (irradiances) cannot be adequately correlated with photosynthetic rates, which respond nonlinearly to irradiance level and variously so for different species and even leaf ages in a mixed canopy. Consequently, a single nonlinear weighting cannot be accurate.
Another important process, photomorphogenesis of stem extension, also responds nonlinearly to irradiance, with high-irradiance sunflecks being very important, if only for their concomitant high red:far red ratio. A single sunfleck is physiologically weighted in the opposite direction from diffuse flux of the same cumulative (time-integrated) irradiance, while averaging sensors weigh the two irradiances identically.
In research on photosynthesis and on canopy light propagation, it is most relevant to measure flux at the leaves as they move rather than at space-fixed points as has been done conventionally. J. M. Norman, C. B. Tanner, "Transient Light Measurements in Plant Canopies," J. Agron, 61, 847 (1969). Furthermore, standard space-fixed sensors only measure irradiance on a fixed and usually horizontal surface. These measurements are difficult to apply to inclined leaf surfaces, as pointed out by C. M. Anderson, "Some Problems of Simple Characterization of the Light Climate in Plant Communities," Light as an Ecological Factor, Blackwell, Oxford, p. 77-99 (1966).
Radiative transfer models for canopies such as that of R. Lemeur, B. L. Blad, "A Critical Review of Light Models for Estimating the Shortwave Radiation Regime of Plant Canopies," Agric. Meteorol., 14, 255 (1974), can display the stochastic variations and average patterns of solar radiation penetration. These models are more sophisticated than conventional measurement techniques. Consequenlty, these models are not readily validated and cannot be relied on for estimating richly varied radiation penetration patterns from measured average irradiances. Moreover, model complexity can be come unmanageable when subsidiary phenomena such as solar tracking by leaves or irregular placement and height of plants are significant. To overcome these problems with conventional measurement systems, it is desirable to develop an improved and accurate solar flux measurement system.